WASHINGTON -- Scientists have unearthed a fossil of a mammal the size of a chipmunk that skittered around with the dinosaurs, with a key feature in the evolution of mammals -- the middle ear bones -- fabulously preserved.

Writing in the journal Nature Wednesday, the scientists said the unusual critter retrieved from a fossil-rich rock formation in northern China provides rare insight into a crucial element of mammalian evolution: ear structure that enabled highly sensitive hearing.

The mammal, named Yanoconodon for the Yan Mountains in China's Hebei Province, lived 125 million years ago during the Cretaceous period, the third and final act of the Mesozoic era, sometimes called the Age of Dinosaurs.

Its body was shaped very oddly for a mammal -- with an elongated torso and short, stubby limbs.

"In a way, it's sort of a salamander-like body form in a mammal," lead researcher Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, said in an interview.

The scientists think Yanoconodon -- 5 inches long and weighing about an ounce -- was nocturnal and ate insects.

It lived in a lush environment with fresh-water lakes, early flowering plants and many other animals. These included a variety of dinosaurs that would have liked nothing more than to make it a furry snack.

Luo said Yanoconodon is particularly important because it displays an intermediate stage in the evolution of mammalian ear structure.

Mammals possess hearing superior to all other vertebrates, and that trait has been fundamental to mammalian life. Many early mammals are thought to have adopted a nocturnal existence that kept them away from the multitudes of dinosaurs and other nasty beasts looking for an easy daytime meal.

Scientists long have searched for clues on the origins of mammalian ear structure. The first true mammals appeared about 220 million years ago, not long after the first dinosaurs, but the process of acquiring the anatomy of fully modern mammals took many more tens of millions of years.

A sophisticated middle ear of three tiny bones called the hammer (malleus), the anvil (incus) and the stirrup (stapes), plus a bony ring for the eardrum (tympanic membrane), give mammals an acute sense of hearing.

Scientists believe these bones evolved from the bones of the jaw hinge in the reptiles from which mammals are thought to have evolved. Luo said the Yanoconodon provided a definitive piece of evidence of this evolution.

The ear bones in Yanoconodon are fully like that of modern mammals, but remain connected to the lower jaw, which is not the case with modern mammals.